reduce,aggregate和groupBy

官網說明：

groupBy:      

This operation may be very expensive. If you are grouping in order to perform an
aggregation (such as a sum or average) over each key, using `PairRDDFunctions.aggregateByKey`
or `PairRDDFunctions.reduceByKey` will provide much better performance.      
    
盡量用 reduce或者 aggregate代替groupBy操作